Optically active hydroxy compounds are valuable synthetic building blocks for the preparation of a multiplicity of pharmacologically important compounds. These compounds are often difficult to prepare by conventional chemical methods and only rarely attain the enantiomeric purity required for pharmacological applications. Therefore, biotechnological methods are usually employed in preparing chiral compounds, the stereoselective reaction being carried out either by whole microorganisms or using isolated enzymes.
The use of isolated enzymes has often proved advantageous here, since higher yields and a higher enantiomeric purity are usually attainable by using such enzymes.
Dehydrogenases and in particular alcohol dehydrogenases are valuable catalysts for obtaining chiral products by stereoselective reduction of organic keto compounds to the corresponding chiral alcohols. Known enzymes are essentially the corresponding enzymes from yeast, equine liver or Thermoanaerobium brockii. These enzymes require NADH (nicotine adenine dinucleotide) or NADPH (nicotine adenine dinucleotide phosphate) as coenzyme. Other examples of known alcohol dehydrogenases are an (S)-specific alcohol dehydrogenase from Rhodococcus erythropolis and an (R)-specific alcohol dehydrogenase from the genus Lactobacillus. Both enzyme types have a broad spectrum of keto compound substrates and have high enantioselectivity. The alcohol dehydrogenases from Lactobacillus kefir (DE 40 14 573) and Lactobacillus brevis (DE 196 10 984) are particularly suitable for obtaining chiral (R)-alcohols.
However, the disadvantages of employing alcohol dehydrogenases are the low enzyme stability and enzyme activity of alcohol dehydrogenases in organic solvents and the frequently only poor water solubility of the keto compounds to be reduced. Another limiting factor for employing alcohol dehydrogenases in organic solvents is furthermore the necessary use of NADP or NAD as cofactor requirement, since the cofactor (NADP, NAD) is water-soluble and is regenerated by economical methods.
It is the object of the invention to improve said disadvantages by modifying the method conditions. This object is achieved according to the invention by using a two-phase system comprising an organic solvent, alcohol dehydrogenase, water, cofactor and keto compound.
The method of the invention has a long stability time due to the enzyme-stabilizing action of the solvent, an enantiomeric purity of more than 99.9% of the prepared chiral hydroxy compounds and a high yield based on the amount of keto compound used.